Interesting that you have 75 V DC under load. With a 2x25V transformer you should get something like 2x33..34 V with no load, and about 2x30..31 V under load.

According to the design guide you need 0.86 K/W for 8 Ohm speakers and 0.05 K/W (!) for 4 Ohm speakers with the 2x37.5 V. Your heatsink doesn't look that efficient.

If you have 75 V and use 4 Ohm speakers you might also reach the current limit.

Concerning the DC voltage at the output and the heat issue at low volume: did you respect the recommendation on page 16 in the datasheet to use 0.1% tolerance resistors for the gain setting (Ri and Rf)?

Heatsinks should have their fins vertical. Think of the air flowing over the fins and imagine where it goes. Mounting the heatsinks like you have will more than double the size required.

If you can't mount them vertically, the next best is to have the heatsink horizontal with the fins pointing up.

Having said that, I think you may have another problem and when you solve that your heatsinks may be OK for normal volumes if you speakers are 8 ohms and reasonably efficient

Did you short the inputs to ground to make DC offset measurements? If you didn't you will find your measurement will drop considerably.

Andrew is talking about the safety earth not ground earth. I measure the resistance between all metal surfaces and the earth wire on the back of the IEC socket. It looks (in your picture) that the front and back panels are metal and are not connect to safety earth. It seems to me people in 230V/240V countries are a lot more concerned about safety earthing issues, rightly or wrongly.

First 75VDC is pushing that top 10% supply range, dissipation goes up as voltage goes up, check Nationals design notes and dissipation figures.
Two the heat sinks are way way to small for a lm4780, again check Nationals app notes on heat sinks.
Third the pot if not separated by a cap will introduce DC into the input. A 4.7uf should not kill your bass at all?? should be smooth to around 5Hz. Get bigger heat sinks, and run the cap between your pot and the amps 1K feeder and 22K shunt, if your designs is built around the data sheets common values.